This invention relates to a rubber mixture containing a nitrile rubber, a metal salt of an acrylate, a liquid acrylate and optionally other additives, a process for its preparation, and use for all types of molded articles, in particular roller coatings.
Rollers having a rubber coating are used in many fields, including the print and textile industry, in machines, such as, for example, fax machines and in the steel industry and paper industry.
Roller coatings are subjected to extreme stresses, particularly in the steel and paper industries. In such applications, the coatings are required to have a combination of the following properties:
low compression set;
low abrasion;
little swelling in the media used;
good resistance to heat and chemicals;
excellent adhesion to the metal surface under the coating; and/or
low build-up of heat under stress.
It has now been found that a rubber mixture which contains a nitrile rubber and, in addition, a metal salt of an acrylate and a liquid acrylate is particularly suitable as raw material for rubber coatings for rollers.
This application accordingly provides a rubber mixture, containing
a) one or more nitrile rubbers
b) one or more metal salts of an acrylate
c) one or more liquid acrylates, optionally applied to a carrier, and
d) optionally other additives and/or fillers.
Nitrile rubbers are meant to be diene-(meth)acrylonitrile copolymers. The preferred dienes here are isoprene and, in particular, butadiene. The copolymers have a content of copolymerized acrylonitrile units and/or methacrylonitrile units of 5 to 60 wt. %, preferably 10 to 50 wt. %.
Moreover, the term explicitly includes hydrogenated nitrile rubbers. According to this invention, xe2x80x9chydrogenated nitrile rubberxe2x80x9d or xe2x80x9cHNBRxe2x80x9d means nitrile rubbers, the Cxe2x95x90C double bonds of which are selectively (that is, without hydrogenation of the Cxe2x89xa1N triple bond) partially or completely hydrogenated. Preferred hydrogenated nitrile rubbers are those having a degree of hydrogenation, based on the Cxe2x95x90C double bonds originating from the butadiene, of at least 75%, preferably of at least 95%, in particular of at least 98%. The degree of hydrogenation can be determined by NMR spectroscopy and IR spectroscopy.
The hydrogenation of nitrile rubber is known: U.S. Pat. No. 3,700,637, DE-A 2,539,132, DE-A 3,046,008, DE-A 3,046,251, DE-A 3,227,650, DE-A 3,329,974, EP-A 111,412, FR-B 2,540,503. Hydrogenated nitrile rubbers are distinguished by having high tensile strength, low abrasion, low residual deformation after compressive stress and tensile load and good resistance to oil, but primarily by a notable resistance to thermal and oxidative influences. Accordingly, hydrogenated nitrile rubbers are preferred in this invention.
Suitable nitrile rubbers generally have Mooney viscosities (DIN 53 523, ML 1+4) of 25 to 100 Mooney units, in particular 40 to 80 Mooney units.
The addition of metal salts of an acrylate to mixtures containing nitrile rubbers is known. Suitable acrylates may be unsubstituted or substituted. Methacrylates are an example of substituted acrylates.
The person skilled in the art knows of suitable acrylates from EP-A1 0,319,320, in particular page 3, lines 16 to 35; from U.S. Pat. No. 5,208,294, in particular column 2, lines 25 to 40 and from U.S. Pat. No. 4,983,678, in particular column 2, lines 45 to 62. Zinc acrylate, zinc diacrylate and zinc dimethylacrylate, in particular, are mentioned there.
It may be advantageous to use the metal salt of the acrylate together with a scorch retarder. Suitable examples here are hindered phenols, such as methyl-substituted aminoalkylophenols, in particular 2,6-ditert.-butyl-4-(dimethylamino)phenol.
The liquid acrylate may be any acrylate which is known to the person skilled in the art and is in liquid form at room temperature.
Trimethylolpropane trimethacrylate (TRIM), butanediol dimethacrylate (BDMA) and ethylene glycol dimethacrylate (EDMA) are particularly preferred.
In order to achieve an improved incorporation, it may be advantageous to use the liquid acrylate bound to a carrier. Examples of suitable carriers are silicates, precipitated silicas, clays, carbon black, talc or polymers. In general, mixtures containing 5 to 50 wt. % carrier are used.
Other suitable additives include those known to the person skilled in the art, for example, vulcanization activators, in particular metal oxides, such as zinc oxide or magnesium oxide, antidegradants, such as alkyl-substituted diphenylamines, mercaptobenzimidazoles, unsaturated ethers, such as Vulkazon(copyright) AFD (Bayer AG, D) or cyclic, unsaturated acetals, such as Vulkazon(copyright) AFS/LG (Bayer AG, D). The following may also be mentioned as additives:
plasticizers, in particular carboxylic esters, as well as sebacic acid and its derivatives or trimellitic acid and its derivatives
processing aids, in particular stearic acid and its derivatives, such as zinc stearate, or polymers, such as poly(ethylene vinyl acetate) (Levapren(copyright) from Bayer AG, D) or poly(ethylene vinyl acrylate)VAMAC(copyright) from DuPont).
It may also be advantageous to introduce fillers into the rubber mixture according to the present invention. These may be non-reinforcing or reinforcing.
Examples of fillers which may be mentioned are:
carbon blacks, such as MT, GPF, SRF blacks and primarily FEF blacks;
metal oxides, such as titanium dioxide (primarily as white pigment);
silicates, such as sodium aluminium silicate;
silicas, in particular precipitated silicas;
To improve the abrasion, so-called active fillers in accordance with the DIN 66131 arc preferred, published, for example, on page 535 of xe2x80x9cHandbuch fxc3xcr die Gummiindustriexe2x80x9d, Bayer AG, 1992, Leverkusen.
Clays, mica, talc.
In addition, it may be advantageous to use extra activators in order to improve the adhesion between filler and rubber, such as, for example, silanes, such as Silquest(copyright) RC-1 (Union Carbide, US). Pigments may also be added.
The quantities of the individual components of the mixture depend on the intended use of the mixture and can be determined by a few preliminary tests.
The materials are generally used in the following quantities (in each case, in phr=per hundred parts of rubber):
metal salt of an acrylate: 10 to 120 phr, preferably 10 to 85 phr, most preferably, 20 to 65 phr,
liquid acrylate: 5 to 80 phr, preferably 20 to 60 phr, in each case calculated without carrier,
antidegradant: 0 to 4 phr,
retarder: 0 to 2 phr,
metal oxides, such as ZnO: 0 to 30 phr,
fillers: 0 to 150 phr, preferably active fillers,
plasticizer: 0 to 20 phr,
processing aids: 0 to 2 phr.
The invention also provides crosslinkable mixtures containing the rubber mixtures according to the present invention, as well as a process for the preparation of the rubber mixtures according to the present invention and of cross-linkable rubber mixtures, characterized in that the components are mixed in a mixing unit.
In order to produce crosslinkable mixtures from the mixtures according to the present invention, crosslinking agents are added to the rubber mixtures according to the present invention. Peroxide systems are suitable crosslinking agents.
Preferred peroxide systems include dialkyl peroxides, ketal peroxides, aralkyl peroxides, peroxide ethers, peroxide esters, such as, for example: di-tert.-butyl peroxide, bis(tert.-butylperoxyisopropyl)benzene, dicumyl peroxide, 2,5-dimethyl-2,5-di(tert.-butylperoxy)hexane, 2,5-dimethyl-2,5-di(tert.-butylperoxy)-3-hexene, 1,1-bis(tert.-butylperoxy)-3,3,5-trimethylcyclohexane, benzoyl peroxide, tert.-butyl cumyl peroxide and tert.-butyl perbenzoate.
The quantities of peroxide are within the range of 1 to 10 phr, preferably within the range of 4 to 8 phr, based on rubber. The cross-linking can be effected at temperatures of 100xc2x0 C. to 200xc2x0 C., preferably 130xc2x0 C. to 180xc2x0 C., optionally at a pressure of 10 to 200 bar. Subsequent to the crosslinking, the vulcanizates can be post-cured by being stored at elevated temperature.
The peroxides can advantageously also be used in polymer-bound form. Appropriate systems are commercially available, for example, Poly-Dispersion(copyright) T (VC) D-40 P from Rhein Chemie Rheinau GmbH, D (=polymer-bound di-tert.-butylperoxyiso-propylbenzene).
The crosslinking can also be achieved by high-energy radiation.
Crosslinking according to the present invention means that less than 10 wt. %, preferably less than 5 wt. %, based on rubber, is extractable during extraction for 10 hours in a Soxhlet attachment with toluene as extracting agent.
The optimal quantity of crosslinking agent is easily determined by preliminary experiments.
Any apparatus for mixing rubbers which is known to the person skilled in the art may be used as the mixing apparatus, in particular internal mixer, open mills and screw-type machines.
Here, care should be taken to ensure that the rubber is not degraded during the mixing procedure. It may be advantageous to cool the mixture during the mixing procedure. In order to avoid scorch, the peroxide is frequently added as the last component, optionally in a separate mixing procedure.
The invention also provides the use of the rubber mixtures according to the present invention for the production of all types of molded articles, in particular coatings for rollers, most particularly rollers for the paper, textiles, printing and steel industries. Rollers generally consist of a metal core having a variable diameter, metal cores with a diameter of 1 to 5 m being preferred. The metal core generally consists of steel of various compositions, onto which the crosslinkable rubber mixture is drawn by means of conventional prior art methods and subsequently cross-linked. A good adhesion of coating to metal and inside the coating is crucial here. It may be advantageous to improve the adhesion of coating to metal by means of adhesion promoters, such as dispersions/solutions of halogenated polymers, optionally with crosslinking agents/fillers/pigments. These substances are commercially available.
The rubber mixtures according to the present invention are particularly suitable for use on rollers, as in the crosslinked condition they combine an excellent hardness in the range above 20 Shore D (DIN 53 505), even at elevated service temperatures, with a good resistance to abrasion, heat, water and chemicals. This balanced range of properties is not achieved in prior art. The rubber mixtures according to the present invention are, of course, also suitable for the production of other molded articles, such as profiles, belts, rings, seals, damping elements, etc.
Accordingly, molded articles, in particular rollers and belts, obtainable using a rubber mixture according to the present invention are also provided by the invention.
To the person skilled in the art, it is a trivial matter to modify specifically the proper- ties of the mixtures according to the present invention by adding other polymers, such as BR, NR, IIR, IR, EPDM, EPM, CR, SBR, AEM, ACM or fluoropolymers.
The following Examples are intended to illustrate the invention, without thereby limiting it.